This invention generally relates to electronic systems and in particular it relates to differential amplifier circuits.
The main distortion source in a power amplifier lies in the output stage. As it supplies the large current variations that the load usually requires, the amplifier open loop gain changes considerably. The main key is the gain of the second stage where the second stage is the output stage. In general feedback theory, as the negative feedback gain that loops around the distortion source increases, the total harmonic distortion (THD) will be reduced by 1+T(w) where T(w) is the loop gain at harmonic frequencies 2w, 3w, etc. Each negative feedback loop around the distortion source has a multiplicative reduction effect.
In a power efficient class AB output stage the current is throttled back as low as possible to save power. In a typical two stage design this implies the second stage gain is severely reduced in a quiescent state and in power conscience designs the second stage gain is typical less than one, and in some cases, when driving very low impedance loads, such as load speakers, the gain can be much less than one. This means there is really only one gain loop around the error source rather than two at low signal levels. The low level linearity (actually in a classical two stage class AB design the entire signal range) is severely impaired by driving small impedance loads.
There are many prior art types of multi-stage amps. These are good for THD because of the multi-loops around the output stage. These provide a multiplication reduction by the loop gains for the closed loop THD of the amp. In a three stage amplifier, even though the last stage gain would be impaired, there are still two gain loops rather than one in the previous case. The major drawback of these prior art designs is the amount of power required to keep stability. When a small impedance load is added to the circuit, since the third stage gain is much less than one in a class AB design, the pole associated with the second stage moves down in frequency. Therefore, to compensate this amplifier, the circuit would either have to pump a high quiescent current to boost the transconductance of the third stage of the amplifier to higher levels, or use a high current to boost the transconductance of the second stage of the amplifier to higher levels. This means that two of the three stages have to be power hungry stages. This is not the answer for low idle current applications when driving low impedance loads (50 ohms or less in standard CMOS). Bipolar amplifiers handle this problem better since they have higher transconductance-to-current ratios than MOS, but the concept of the problem remains.
A system feedback loop method with low current and improved distortion performance is disclosed in U.S. Pat. No. 6,275,102 xe2x80x9cDistortion Correction Loop for Amplifier Circuitsxe2x80x9d granted on Aug. 14, 2001. It is based on single-ended, or pseudo-differential structures that accept a differential signal, but the output is single-ended. Duplicating these structures for the opposite phase gives the xe2x80x9cpseudoxe2x80x9d differential device, but it is not a true fully differential circuit.
The differential circuit with linearity correction loop includes a main differential amplifier, and a correction amplifier having inputs coupled to the outputs of the main differential amplifier through feedback paths. The output signals from the correction amplifier are combined with the inputs to the main amplifier such that a negative feedback loop is formed around the differential circuit. This feedback loop provides stability with only a minor power increase.